The present invention relates generally to ring laser gyroscopes, and more particularly, relates to a method of preventing electrode seal degradation.
A ring laser gyroscope detects and measures angular rates by measuring the frequency difference between two counter-rotating laser beams according to the Sagnac effect. The two laser beams simultaneously circulate in the cavity of the gyroscope. Mirrors are used to reflect each beam around the cavity. The two laser beams will ideally have identical frequencies when the sensor is at rest. If the sensor is rotated, the beams will have different frequencies. This frequency difference is measured to provide the rate of rotation.
Gyroscopes are used in navigation, stabilization, guidance, and control applications and are located in aircraft, boats, tanks, pipelines, and missiles. The applications generally fall within one of two categories, single or short term use, and continuous or long term use. An example of a single use application of a ring laser gyroscope is a missile application. The gyroscope guides the missile to its target and is destroyed upon impact. This type of gyroscope has an operational lifetime that may be measured in minutes and is not exposed to the harsh operating conditions that the continuous use gyroscopes are. The single use ring laser gyroscopes are generally smaller and manufactured with different materials than continuous use gyroscopes.
An example of a continuous use application of a ring laser gyroscope is an aircraft application. The operational lifetime of a ring laser gyroscope on a commercial airplane may be ten to twenty years. The gyroscope is exposed to extreme temperature and pressure fluctuations over an extended period of time. Because of the extreme conditions in which a continuous use gyroscope may be operated, the frame of this type of gyroscope must be manufactured using materials that are resistant to expansion over a wide temperature range. One such material is Zerodur, a glass ceramic material with an extremely low co-efficient of thermal expansion.
One of the problems with using this type of frame material is that it tends to have a higher ionic conductivity value than other dielectric materials. These frame materials contain alkali ions that are highly mobile in the presence of an electrical field. The ions are attracted to the cathode mounted on the frame of the gyroscope because it is at the lowest electrical potential due to the typical method of applying power to the gyroscope. The migration of the alkali ions to a cathode will cause an ion-rich layer to be deposited on the seal located between the cathode and the frame.
Indium is frequently chosen as a seal material because of its unique properties of adhering to both ceramics and metals, and of not losing its vacuum seal in the presence of thermal expansion. For the proper operation of the gyroscope, this seal must not degrade allowing the lasing gas to escape. Therefore, there is a need to prevent the degradation of the seal.
U.S. Pat. No. 5,856,995, xe2x80x9cRing Laser Gyroscope with Ion Flux Trap Electrode,xe2x80x9d described a method of trapping the ions before they migrate to the cathode. This method requires an electrode ring to be placed in direct electrical contact with the frame surrounding the cathode as seen in FIG. 1. The electrode ring has a more negative electric potential than the cathode. This will attract the ions to the ring, and not to the cathode, preventing the ions from degrading the seal. The electrode ring may be made of a thin sheet of copper attached with a conducting adhesive, a thin metal film applied by vacuum deposition, a machined metal alloy attached by mechanical means, or conductive ink brushed onto the frame surface.
Another technique is set forth in U.S. Pat. No. 6,025,914, xe2x80x9cCathode Seal Including Migration Resistant Dielectric Materialxe2x80x9d, which is assigned to the same assignee as the present invention. This method adds a dielectric barrier material between the gyroscope frame and the cathode as seen in FIG. 2. The dielectric barrier material layer reduces the electric field formed in the gyroscope frame, and thus reduces the ion migration. The dielectric barrier may be formed by vapor deposition or welding a sheet of material between the cathode and the frame.
It would be desirable to prevent an ion layer from forming on an electrode seal without modifying the manufacturing process of a ring laser gyroscope.
An exemplary embodiment is described for using current control biasing to protect electrode seals on a ring laser gyroscope. The seals are located between the frame of the gyroscope and each of the electrodes. When the gyroscope is energized, ions in the frame will generally migrate towards the lowest electrical potential. If one of the electrodes is at the lowest electrical potential, the ions will form a layer on the electrode seal causing it to degrade. By providing a positive supply voltage and locating the current control on the non-ground side of the power supply, the mounting structure will be at the lowest electrical potential.